One type of conventional microelectromechanical systems (MEMS) gyroscope includes a proof mass suspended by springs within a support structure. A drive circuit causes the proof mass to oscillate (translate back and forth) in a plane of oscillation. Hence, this type of device is often referred to as a “shuttle-type gyroscope.” When the support structure is rotated in the plane of oscillation, the proof mass tends to continue oscillating in the plane but, as a result of the Coriolis Effect, the proof mass is displace in a direction perpendicular to both the axis of oscillation and the axis of rotation. This displacement causes a change in capacitance between the proof mass and a stationary electrode. The magnitude of the change in capacitance is proportional to the angular rate of the rotation. Shuttle-type gyroscopes operate at relatively low frequencies (apx. 3-30 kHz), and they have relatively large form factors and require relatively high operating voltages and low pressure packaging environments.
Capacitive “bulk acoustic wave” (BAW) silicon disk gyroscopes were developed to alleviate some of the disadvantages of shuttle-type gyroscopes. In a BAW gyroscope, a bulk mass, typically supported by a pedestal, is made to oscillate in a “bulk acoustic mode.” Instead of the entire bulk mass translating back and forth as in the shuttle-type gyroscope, the crystal lattice of the bulk mass oscillates. As a result, the shape of the bulk mass oscillates, typically at a frequency of several megahertz. For example, a bulk mass disk may oscillate between two oblong shapes, one oriented such that its major axis is perpendicular to the major axis of the other oblong shape.
When a BAW gyroscope is rotated, the bulk mass responds by oscillating between two other shapes or two other axis orientations. The amplitude of this second oscillating mode is proportional to the angular rate of rotation. Stationary electrodes capacitively detect the amplitude of the second oscillation. A drive circuit drives the BAW gyroscope to oscillate between the first two shapes (referred to as a “drive mode” of oscillation), and the second oscillation (resulting from the rotation) is referred to as a “sense mode” of oscillation.
BAW gyroscope use has increased in recent years, at least in part because a typical BAW gyroscope can provide a higher gain (i.e., an increased output signal level for a given rate of angular rotation) and require less power than shuttle-type gyroscopes. However, very slight manufacturing or material imperfections (“non-idealities”) in BAW gyroscopes pose serious problems. For example, such non-idealities cause the resonant frequency of the drive mode of oscillation to be different than the resonant frequency of the sense mode of oscillation, leading to a loss of sensitivity (i.e., lower amplitude of oscillation in the sense mode for a given angular rate of rotation) and, more seriously, in phase errors, which cause an output from the gyroscope in the absence of rotation.
The prior art compensates for non-idealities by applying DC bias voltages to “tuning” electrodes located near the bulk mass, based on measured non-idealities of a given device. These compensations are applied in an attempt to cause the drive and sense modes to have identical resonant frequencies, i.e., for the drive and sense modes to be degenerate. However, maintaining degeneracy is difficult in the case of high quality (Q) oscillators, due to the need for tight control of these parameters over time and despite variations in temperature of the bulk mass.